Here's why: Being out in the sun without proper protection from ultraviolet light exposure can increase your risk for sunburn, wrinkles, and skin cancer, the most common type of cancer in the U.S., with more than 3.5 million cases diagnosed every year.

But if you are hazy on sunscreens and how to use them, your health could be on the line.

There are two types of ultraviolet light: UVB, which causes sunburn and can lead to skin cancer, and UVA, which penetrates the skin more deeply and can cause wrinkles and skin cancer.

Dermatologists Jennifer Stein, MD, PhD, assistant professor of dermatology at New York University Langone Medical Center in New York and James Spencer, MD, a doctor in private practice in St. Petersburg, Fla., and a member of the American Academy of Dermatology, clear up myths about sunscreen.

Sunscreen Myth: I can skip it.

Maybe you think you can pass on sunscreen because you don't bask in the sun. But sunscreen is not just for sun worshippers.

"If you're going to be outdoors, you should wear sunscreen, even when it's cloudy outside. You can still get sunburn through cloud cover," Stein says.

"People with darker skin are definitely less like to burn, but they can still burn and should wear some form of sunscreen that protects against UVA and UVB," Stein says.

If you've got a tan, either from the sun or a tanning bed, "it means your skin is damaged," Stein says. "A tan may give you a little bit of protection, but you can still burn."

"There's a myth out there that you should tan before going on vacation because it will protect you from getting burned and that's just not true," Stein says. "Also, the tan you get from a tanning bed doesn't protect you. It's a different kind of tan because it [is from] high amounts of UVA, which darkens the skin quickly."

And if you skip sunscreen because you don't like how it feels on your skin, shop around. "There are so many sunscreens on the market," Spencer says. "Don't give up."

Got sensitive skin? "You can try ones marked 'sensitive skin,' which often are the ones that have a physical blocker in them," such as titanium dioxide or zinc oxide, Stein says. Spencer agrees. "People with sensitive skin tend to do better with the physical blocks," he tells WebMD.

She notes that sunscreens labeled for babies or children are often the same as the "sensitive skin" versions of those products, just repackaged for a different age group. Stein's other tips for people with sensitive skin:

Do a test spot. "If you're concerned about a new sunscreen, you can first try it on the inside of your arm before you use it on your face or put it all over your body," Stein says.

Wear protective clothing. "Clothing and a hat is even better than sunscreen. The more covered up you are, the less sunscreen you need," Stein says.

Sunscreen Myth: The SPF in my makeup is enough.

Many women may rely on sunscreen in their makeup. But you might need more than that.

"If you use [foundation], a few spots of sunscreen on your face isn't going to be enough out in the sun," Stein says.

"You should wear at least an SPF of 30," Stein says. "The easiest approach is to use a facial moisturizer that already has sunscreen in it."

Sunscreen Myth: All sunscreens are the same.

Not so. Sunscreens can differ in the way they protect your skin. Some are physical sunblocks, which use zinc oxide or titanium dioxide to block UVA and UVB rays. Others use chemicals such as avobenzone to do the job.

Newer active ingredients include Helioplex and Meroxyl SX.

"Dermatologists like Helioplex and meroxyl because these ingredients are photostabilized, which means they give you good UVA and UVB protection, and they're more stable so they won't break down as quickly," Stein says.

What offers the best protection? That's a matter of debate.

The Environmental Working Group has reported that some sunscreen products don’t adequately protect the skin, but the Personal Care Products Council, an industry group, has disputed that. Consumer Reports also reviews and rates sunscreens.

According to the American Academy of Dermatology, you should look for a sunscreen that has an SPF of 30 or higher that provides broad-spectrum coverage against both UVA and UVB light.

The FDA allows sunscreens to put "Broad Spectrum" on their labels if they pass the FDA's test for protection against both UVA and UVB rays. The FDA states that "sunscreens labeled as both Broad Spectrum and SPF 15 (or higher), if used regularly, as directed, and in combination with other sun protection measures will help prevent sunburn, reduce the risk of skin cancer, and reduce the risk of early skin aging." Broad-spectrum sunscreens with SPF of less than 15 will be required to have a warning stating that the product has not been shown to help prevent skin cancer or early skin aging. Those rules take effect in the summer of 2012, but sunscreen makers can start following them sooner than that.

Sunscreen Myth: A little sunscreen will see me through the day.

"The general principle is to reapply every two to four hours," Spencer says. "Sunscreen does go away with time."

Don't be stingy when you're putting it on yourself or your children.

"To cover your whole body, you would have to fill a shot glass," Stein says. "A good way to conserve sunscreen is to cover up with clothing. Clothes are more reliable than sunscreen -- you don't have to worry about forgetting about it or reapplying it."

If you get into the water, you may need to reapply more often.

The FDA doesn't allow sunscreen makers to claim that their products are "waterproof" or "sweatproof," or identify their products as "sunblocks," because the FDA says those claims overstate their effectiveness. Sunscreens can claim that they are "water resistant," but they have to specify how long that lasts.

You may also want to check on whether your prescriptions make your skin more sensitive to the sun.

"Certain blood pressuremedications can make your skin more sensitive to the sun and so can some antibiotics, such as doxycycline, which is an oral antibiotic used to treat acne. Be sure to talk to your doctor about this," Stein says.

"The ears and the back of the neck are commonly neglected," Stein says. "You can actually get sunburn on your scalp, so wearing a hat is a good way to get shade on your face and that will give you good face protection."

Don't forget about your lips. The American Academy of Dermatology recommends wearing a lip balm with an SPF of at least 30.

"There are no real major differences; these are just vehicles for the sunscreen and it depends on what the consumer likes," Spencer says.

"Men often do better with alcohol-based sprays because they don't like greasy products. Women often do better with lotions and creamier products because they like the moisturizing effect," Spencer says. "There are many different sunscreen products to choose from. What's most important is compliance -- if you like the product, you're more likely to use it."

Whatever kind of sunscreen you choose, put it on dry skin 15-30 minutes before you go outside, according to the American Academy of Dermatology.

Sunscreen Myth: Last year's bottle is still OK.

"You should use enough so that you're not using the same bottle summer after summer. If you're doing it right, you're not going to have leftovers next year," Stein says.

Check the expiration date on your sunscreen bottle.

"Some sunscreens break down quickly, especially the ones that give you UVA protection. So it shouldn't sit in your bathroom cabinet for too long," Stein says.

Spencer discloses that he has consulted for L'Oreal. Stein has no disclosures.